Problem: Kevin is $2$ times as old as Gabriela. $12$ years ago, Kevin was $6$ times as old as Gabriela. How old is Kevin now?
We can use the given information to write down two equations that describe the ages of Kevin and Gabriela. Let Kevin's current age be $k$ and Gabriela's current age be $g$. The information in the first sentence can be expressed in the following equation: ${k = 2g}$ Twelve years ago, Kevin was $k - 12$ years old, and Gabriela was $g - 12$ years old. The information in the second sentence can be expressed in the following equation: ${k - 12 = 6(g - 12)}$ Now we have two independent equations, and we can solve for our two unknowns. Because we are looking for $k$, it might be easiest to solve our first equation for $g$ and substitute it into our second equation. Solving our first equation for $g$, we get: ${g = \dfrac{k}{2}}$. Substituting this into our second equation, we get: $ {k - 12 = 6 (}{\frac{k}{2}} {- 12)} $ which combines the information about $k$ from both of our original equations. Simplifying the right side of this equation, we get: $k - 12 = 3 k - 72$. Solving for $k$, we get: $2 k = 60$. $k = \dfrac{1}{2} \cdot 60 = 30$.